1. Field of the Invention
The present invention relates in general to a luminaire arrangement and in particular to a guard for protecting a light source of the luminaire arrangement.
2. Description of the Prior Art
In many instances luminaires are employed in locations where it is desirable to protect the luminaire from foreign objects. For example, in school rooms it is often necessary to use a guard in association with the luminaire to avoid paper wads, erasers, and other foreign matter from being thrown into the luminaire.
Heretofore, it has been customary to provide a wire guard for use with such a luminaire. A conventional guard is typically made with radial wire spokes and circumferential wires welded together where the wires cross. The guard is typically positioned such that the circumferential wires circumscribe a portion of the light source or its support. These known guards are useful in a variety of applications, one suitable application being in situations where the luminaire is for indirect lighting. When a light source having a bulb which is coated with a light diffusing material is employed, such known wire guards do not tend to cast sharp distinct shadows on the ceiling. This is because the coated light source acts as a relatively large luminous body, as compared to a point or line source. As such, the light source emits light rays from a relatively large surface area. Hence, if an obstruction which is narrower than the source, e.g. a wire-like element, is positioned in close proximity to the source much of the light will pass beyond the obstruction and illuminate the surface which would otherwise be eclipsed by the obstruction, .i.e., the surface directly behind the element.
Applicant has found, however, that when a long and narrow or thin light source, e.g., a clear linear light having a clear protective bulb with an exposed thin linear arc of light, is used in a luminaire, there may be a shadow problem. More particularly, where an obstructing element is aligned in a common plane with the linear light source, and where the linear light source, i.e., the arc of light is thinner or at least not significantly thicker than the obstruction, light will not pass the obstruction in the manner discussed above. Instead, a very sharp shadow will be cast on the surface illuminated.
If the arc stream of the linear light source is oriented in the vertical direction and is a very fine line of light (e.g. 1/8 inch or less by 13/4 to 4 inches) the arc stream is long enough to bridge across the circumferential wires of a guard circumscribing the source so no appreciable shadow is cast by them. In other words, much of the light from the source will pass behind these circumferential wires to illuminate the area which would otherwise be shadowed. However, if the radial wires are positioned so as to lie within a vertical plane including the arc stream in its entirety, sharp and distinct radial shadows result.
The term "element" as used herein and in the claims refers to any segment of wire-like material in the guard which is capable of lying entirely within at least one plane, i.e., a segment which projects at most in two directions. The thickness of the element, i.e., its cross-sectional dimension, is discounted. Where the element is linear, the element will lie in a plurality of planes. If a segment of wire-like material is three-dimensional (not counting its thickness) then such a segment is considered to include more than one element. That is, even if the segment is a continuous piece of material, if it projects in directions along the X, Y, and Z geometry axes, then the segment is divided into smaller units, i.e., elements, each of which lies within at least one plane.